Fault tolerance and cross client compatibility

Greg Whalin gwhalin at meetup.com
Tue Apr 12 06:03:13 PDT 2005

This is a bug in the java client and a nice catch.  I have fixed the 
code in head release of repository to have fix to match perl way of 
doing things.  Not tested, but should work.

Mathias Herberts wrote:
> Hi,
> we intend to use memcached as a way of passing requests results
> between a producer and a consumer. For fault tolerance reasons we
> intend to use several memcached servers and rely upon the rehashing
> capabilities of client APIs.
> The producer is written in Perl and the consumer in Java, so I looked
> at both client implementations and found out that both rehashing
> techniques are incompatible.
> Has anybody else experienced this and come up with patches to render
> the rehashing algorithms compatible?
> The code in Java reads:
>                         // if we failed to get a socket from this server
>                         // then we try again by adding an incrementer to the
>                         // current hash and then rehashing 
>                         hv += ("" + hv + tries).hashCode();
> that is the hashCode function used is the native String one and not
> the one choosen earlier (NATIVE, COMPAT, etc).
> The code in Perl reads:
>                        $hv += _hashfunc($tries . $real_key);  #
> stupid, but works
> the rehashing is not done on the same data.
> I think this part would need to be normalized somehow so different
> client APIs will behave the same way in case of a server failure (of
> course erratic behaviour might still be observed in case of network
> partitioning but this is a different failure scenario).
> Mathias.

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